A recently developed, gain-controllable amplifier whose gain is controlled by adjusting a D.C. bias voltage dispenses with shield wires between the amplifier and the variable resistor for controlling the D.C. bias voltage without any fear of crosstalk of electric signals. The conventional gain-controllable amplifier comprises a differential amplifier stage. An input signal is applied to the commonly connected emitters of the transistors of the differential amplifier, and the bias potentials applied to the bases of the respective transistors in the differential amplifier are varied manually, with the results in variation of the collector currents of both the transistors and in variation of gain of the output derived from the collectors of the transistors. For varying base bias potential, there have been proposed two types of attempts. One of these uses a circuit including a transistor with the base on which a potential divided by fixed and variable resistors is to be impressed, and a resistor connected to the emitter of this transistor, so that a base bias potential for the differential amplifier is derived from the emitter. Another of these uses another differential amplifier having load transistors, whose bases receive a constant potential, and whose emitters are connected to the respective collectors of another differential amplifier. The base of one of the transistors of another differential amplifier is supplied with a constant potential, while the base of the other is supplied with a potential controlled by a resistor divider. The output potential of the transistors of another differential amplifier receiving the input signal via base-emitter junction of additional transistors.
In the former case, a variable resistor is used as a resistance-dividing means for controlling a potential on the base of a transistor, thereby varying a dividing ratio of the power voltage. However, this resistance dividing means is attended with a base voltage whose value abruptly changes at a given value, the aforesaid base voltage being interrelated to the variation in resistance of the aforesaid variable resistor. As a result, adjustment of gain may be feasible only within a small allowable range of variation in resistance. On the other hand, in the latter case, gain varies linearly or in proportion of the variation in resistance across the dividing point thereof, thus permitting easy adjustment of the gain, while suffering from a narrower range of the gain adjustment.